This application requests support to determine the Fraction Size Equivalent Dose (FED) levels for the intensity modulated radiotherapy (IMRT) treatment planning process of benign and biopsy only low grade brain neoplasms that lead to a low impairment in the specific neurological functions of passive language comprehension (hearing), language, fine motor control, and visual perception (vision), and therefore allow for sparing of these areas during optical guided conformal avoidance radiotherapy. We are hypothesizing that the use of functional magnetic resonance imaging (fMRI) in conjunction with neurological assessment, will result in a better understanding of Fraction Size Equivalent Dose (FED) levels (either maximum or mean FED), obtained from function specific dose volume histograms which lead to the impairment of a particular neurological function. Using the FED formalism one can reconvert BED values back to biologically equi-effective doses delivered at some standard dose-per-fraction, producing numbers that are more directly comparable with the dose-levels delivered using typical external beam schedules. Allowable FED levels that lead to minimal impairment of a particular neurological function could then be used in the design of image guided conformal avoidance radiotherapy treatment plans. The proposed research is based upon extensive preliminary data indicating that fMRI adds additional information about the location of eloquent volumes of brain responsible for the specific neurological functions (passive language comprehension (hearing), language, fine motor control, and vision) that can be incorporated into a radiation therapy treatment plan, with the goal of decreasing radiation injury to these areas. The Specific Aims of the project are: 1) To evaluate and quantify the reproducibility of fMRI maps of eloquent brain areas corresponding to specific neurological functions based on activation maps obtained with different thresholds in patients with benign and biopsy only low grade brain neoplasms. 2) To assess the impairment in neurological function in image guided intracranial radiotherapy using neurocognitive assessment tools and to derive dose response curves relating the impairment in a particular neurological function to the FED received by the area of eloquent brain corresponding to it.